In a turbine engine, fitted platforms for fan blades need to perform several functions. From an aerodynamic point of view, such platforms have the main function of defining the air flow passage. They must also be capable of withstanding large forces without deforming and while remaining secured to the disk that carries them.
In order to satisfy these various requirements, certain configurations have been proposed in which platforms possess a first portion serving to define the air flow passage and to retain the platform while the engine is rotating, and a second portion serving to limit deformation of the first portion under the effect of centrifugal forces and to hold the platform in position when the engine is stopped.
In existing solutions, the platform may be in the form of a box with a two-dimensional flow passage wall that is held downstream by a drum and upstream by a shroud, the shroud being held upstream over the tooth of the fan disk (a flange of the shroud serving to block the upstream end of the platform both axially and radially).
Such upstream retention performed over the tooth of the disk with a shroud presents the drawback of imposing a large hub ratio, where the hub ratio is the ratio of the radius measured between the axis of rotation and the point on the leading edge of the blade that is flush with the surface of the platform divided by the radius measured between the axis of rotation and the outermost point of the leading edge.
In order to optimize the performance of the fan, and more generally of the turbine engine, it is desirable to have an assembly comprising a platform that is fitted to the fan blade mounted on a fan disk that presents a hub ratio that is as small as possible.